Adhesion promoting additives and low temperature curig organosiloxane compositions containing same

Abstract: Adhesion promoting additives for organosiloxane compositions that cure by a platinum-catalyzed hydrosilation reaction are reaction products of (1) an ethylenically unsaturated ether of a polyfunctional alcohol and (2) at least one silicon compound selected from (i) epoxysubstituted organosilicon compounds containing at least one hydrolyzable group per molecule and (ii) tetraalkyl orthosilicates, also referred to tetraalkoxysilanes, where the reaction is conducted in the presence of a suitable catalyst such as an organotitanium compound. The additive optionally includes a chelated organoaluminum compound.

Description: This invention relates to organosiloxane compositions that cure by a platinum-catalyzed hydrosilation reaction and, more particularly, to pre-reacted combinations of additives that improve the adhesion of organosiloxane compositions to a variety of substrates, including metals and plastics, at curing temperatures below 100°C. The curing characteristics of these organosiloxane compositions and their properties as cured materials are desirable for a variety of end use applications, particularly in the electrical and electronics areas. The cured materials are useful as protective coatings, potting materials and encapsulants for integrated circuits and other electronic devices or as coatings for printed circuit boards. A shortcoming of such organosiloxane compositions is their inability to adhere strongly to many substrates, particularly plastics and certain metals. One method for remedying this deficiency is the use of primer compositions or adhesion promoting additives containing silanes and organosiloxanes with a plurality of silicon-bonded hydrolyzable groups and at least one organofunctional substituent that is bonded to silicon through at least one carbon atom. Primer compositions are applied to substrates to which adhesion is desired prior to application of the curable organosiloxane composition. Adhesion promoters are also present in the curable organosiloxane composition. Examples of hydrolyzable groups that are present in known adhesion-promoting organosilicon compounds are alkoxy, carboxy such as acetoxy, ketoximo such as methyl-ethyl ketoximo and amido. The organofunctional group is typically one that will react with the substrate to which adhesion is desired. Examples of suitable organofunctional groups include alkenyl such as vinyl and allyl, aminoalkyl, epoxyalkyl, mercaptoalkyl and (meth)acryloxyalkyl. Three classes of silanes frequently used as adhesion promoting additives are epoxy-functional silanes such as 3-glycidoxypropyltrialkoxysilanes, alkenyl-functional silanes such as vinyltrialkoxysilanes and alkyl silicates such as tetraethyl orthosilicate. These silanes can be at least partially replaced by their hydrolysis/condensation products. The use of organic or organosilicon compounds containing epoxy groups in combination with organic aluminum or zirconium compounds as an adhesion promoter for organosiloxane compositions that cure by a platinum catalyzed hydrosilation reaction is known in the art. See, for example, EP-A 0,240,333 and JP-A 1,503,369. The art also teaches using two or more organo-functional groups, with or without alkoxy, to achieve cohesive bonding between silicone elastomers cured by a platinum-catalyzed hydrosilation reaction and various substrates. These organofunctional groups can be present on the same or different organosilicon compounds. This art is represented by EP-A 0,493,791; US-A 4,087,585; US-A 5,270,425; US-A 4,732,932; US-A 4,243,718; US-A 4,912,188; US-A 4,891,393; EP-A 0,460,698; US-A 5,082,894; US-A 4,719,262; US-A 5,106,933; CA-A 1,287,434; JP-A 54/13,454; US-A 4,243,718; US-A 4,786,701 and US-A 5,023,288. A disadvantage of many organofunctional organo-silicon compounds used as adhesion promoters in prior art organosiloxane compositions, particularly the combination of epoxy- and vinyl-functional organosilicon compounds described in US-A 4,087,585, is their adverse effect on the cure rate and completeness of cure, particularly at temperatures below 120°C. Some integrated circuits and other heat-sensitive electronic devices cannot be exposed to temperatures above this limit even for relatively short periods of time without risking damage to the device. This invention provides a group of reaction products derived from organosilicon compounds that impart adhesion to curable organosiloxane compositions at curing temperatures below 100°C. This invention also provides a class of organosiloxane compositions that cure at temperatures below 100°C. to form adherent coatings on heat-sensitive electronic devices and circuit boards containing these devices. The adverse effects of the adhesion promoting additives described above regarding US-A 4,087,585 on the storage stability, curing properties and/or adhesion of curable organosiloxane compositions are eliminated when the adhesion promoting additive is a reaction product of (1) an ethylenically unsaturated ether of a polyfunctional alcohol and (2) at least one compound selected from (i) epoxy-substituted organosilicon compounds containing at least one hydrolyzable group per molecule and (ii) tetraalkyl orthosilicates, also referred to as tetraalkoxysilanes. The reaction product is prepared in the presence of a suitable catalyst such as an organotitanium compound. The adhesion to certain substrates is further improved when the reactants used to prepare the composition include at least one member from both of the present classes of silicon compounds, referred to as (i) and (ii) above. This invention introduces an adhesion-promoting additive for curable organosiloxane compositions, said additive comprising the reaction product of (1) a partial ether derived from an ethylenically unsaturated alcohol and a polyhydric alcohol, (2) at least one organosilicon compound containing at least one epoxide group and at least one silicon-bonded hydrolyzable group, and (3) at least one tetraalkyl orthosilicate; where said reaction product is prepared in the presence of a catalytically effective amount of an organotitanium compound. This invention also provides curable organosiloxane compositions exhibiting adhesion to both organic and inorganic substrates following curing at temperatures below 100°C., said compositions comprising (A) a polyorganosiloxane containing at least two alkenyl radicals per molecule, (B) an amount sufficient to cure said composition of an organohydrogensiloxane containing at least two silicon-bonded hydrogen atoms per molecule, where the sum of the alkenyl radicals per molecule of (A) and the silicon-bonded hydrogen atoms per molecule of (B) is greater than 4, (C) an amount sufficient to promote curing of said composition of a hydrosilation catalyst selected from metals from the platinum group of the periodic table and compounds of said metals, and (D) an amount sufficient to achieve adhesion to substrates in contact with said composition during curing at temperatures below 100°C. of an adhesion promoting additive comprising the reaction product of (1) a partial ether derived from an ethylenically unsaturated alcohol and a polyhydric alcohol and (2) at least one organosilicon compound containing at least one epoxide group and at least one silicon-bonded hydrolyzable group, and (3) at least one tetraalkyl orthosilicates, where said reaction product is prepared in the presence of a catalytically effective amount of an organotitanium compound. This invention also introduces a method for coating substrates using the present compositions and for adhering two substrates together by applying a layer of the present curable compositions to a surface of at least one of the substrates, placing the coated surface in contact with a surface of a second substrate and maintaining the surfaces in contact during curing of the composition. A particular feature of the present organosiloxane compositions is the adhesion promoting additive. This additive comprises the product obtained by reacting in the presence of an organotitanium catalyst a partial ether derived from an ethylenically unsaturated alcohol and polyhydric alcohol containing at least one unesterified hydroxyl group and at least one organosilicon compound selected from silanes containing (a) at least one epoxy-substituted group that is bonded to silicon through an alkylene radical and (b) at least one hydrolyzable group per molecule and tetraalkyl orthosilicates. To achieve adhesion to a wider variety of substrates at lower curing temperatures, the ingredients used to prepare the present reaction products include at least one silicon compound from each of the groups identified as (D)(2)(i) and (D)(2)(ii) and the reaction products are used in combination with a chelated aluminum compound. While not wishing to be bound to any theories, it is believed that the active adhesion promoting species are reaction products of the hydroxyl group of the partially etherified polyhydric alcohol with one of the alkoxy or other hydrolyzable groups present on the epoxy-functional organosilicon compound and one of the alkoxy groups present on the tetraalkyl orthosilicate. Reactant D1 used to prepare the present adhesion promoting additives is a partial ether derived from an ethylenically unsaturated alcohol and an alcohol defined as polyhydric because the free, i.e unetherified, alcohol contains at least two carbinol (≡COH) groups. At least one but not all of these carbinol groups is reacted to form an alkenyloxy group such as propenyloxy, also referred to as an allyl ether group. The alkenyloxy group can contain from 3 to 8 carbon atoms and is preferably terminally unsaturated. Allyloxy groups are preferred, based on the availability of the corresponding starting materials. The polyhydric alcohol can contain from 2 to 10 carbon atoms. Preferred polyhydric alcohols include diols such as ethylene and propylene glycols and 1,4-butanediol, triols such as glycerol and trimethylolpropane (3,4-di-hydroxymethyl-1-butanol) and compounds containing 4 or more hydroxyl groups, such as pentaerythritol. The epoxy-substituted organosilicon compound can be a silane or a polyorganosiloxane. This compound contains at least one epoxy group that is bonded to a silicon atom by means of an alkylene radical containing at least 1 carbon atom. Alternatively, the epoxide group can be part of a carbocyclic ring that is bonded to a silicon atom through an alkylene group. This reactant also contains at least one hydrolyzable group. It is preferably a silane represented by the formula R 1 SiR 2 m R 3 (3-m) where R 1 represents an epoxyalkyl radical or an epoxy-substituted cycloalkyl radical, R 2 represents an alkyl radical, R 3 represents a hydrolyzable group and m is 0 or 1. Examples of suitable hydrolyzable groups represented by R 3 include alkoxy, carboxy, i.e. R'C(O)O- and ketoximo. In preferred embodiments, the hydrolyzable groups are alkoxy groups containing from 1 to 4 carbon atoms. Numerous epoxy-functional silanes are described in US-A 3,455,877, which discloses epoxy-substituted alkoxysilanes that are suitable for use in the present compositions. The epoxy-functional silane is preferably a mono(epoxyhydrocarbyl)trialkoxysilane wherein the carbon atoms of the epoxy group are separated from the silicon atom by an alkylene radical. We prefer a 3-glycidoxypropyltrialkoxysilane where the alkoxy group is methoxy or ethoxy for this ingredient. Examples of suitable epoxy-substituted silanes include 3-glycidoxypropyltrimethoxysilane and 3,4-epoxycyclohexylmethyltrimethoxysilane. The second class of silicon compounds used as reactants to prepare the present adhesion additives includes tetraalkyl orthosilicates, also referred to as tetraalkoxysilanes. Examples of suitable compounds include tetramethyl orthosilicate, tetraethyl orthosilicate and tetra-n-propyl orthosilicate. The adhesion-promoting additives of the present invention are prepared by reacting the partial ether of an ethylenically unsaturated alcohol the organosilicon compound and the tetraalkoxysilane in the presence of a suitable catalyst that promotes a hydrolysis/condensation reaction involving the ingredients. The reaction product is believed responsible for the development of adhesion that occurs during curing of the present organosiloxane composition at temperatures no higher than 100°C. and preferably below 85°C. The comparative examples herein demonstrate that satisfactory adhesion, as determined by the percent cohesive failure between the cured organosiloxane composition and the substrate, is not achieved if the ingredients of the present adhesion promoting additive are not reacted prior to being added to the curable organosiloxane composition. Each of the reactants constitutes at least 10 percent of the combined weight of all reactants present. The high level of cohesive failure that characterizes the present compositions is not achieved when this limitation is not observed. In addition to the above reactants, the reaction mixture used to prepare the present adhesion promoting additives contains an amount of an organotitanium compound sufficient to function as a catalyst for the reaction. Preferred catalysts for preparing the present adhesion promoting additives are organotitanium compounds such as tetraalkyl titanates and alkyl titanates containing a chelated organic structure such as that derived from 1,3-diketones and beta-keto esters such as actylacetoneates. The chelating agent can be any organic compound capable of forming at least two coordinate bonds with a titanium atom, resulting in a cyclic structure. Preferred chelating agents are 1,3-diketones such as acetylacetone. The concentration of the organotitanium compound is typically from 0.1 to 5 weight percent, based on the combined weight of the other reactants. The use of an excessive amount of organotitanium compound may cause the viscosity of the curable composition to increase during storage. Our adhesion promoting additives optionally include a chelated aluminum compound that improves the adhesion between the present compositions and a variety of substrates, with which the compositions are in contact during the curing reaction. The aluminum compounds are derivatives of chelated organic compounds. Suitable chelating agents are discussed in this specification infra in connection with the organo-titanium compound. Preferred chelating agents are 1,3-diketones such as acetylacetone. The concentration of the chelated aluminum compound is typically from 0.02 to 0.1 weight percent, preferably 0.05 weight percent, based on the weight of the curable organosiloxane composition. The use of excessive amounts of the aluminum compound can result in gas evolution during curing of the compositions, which may affect the properties and appearance of the final cured product. The polyorganosiloxane, ingredient A, of this invention is a principal ingredient of these compositions. It must contain at least two silicon-bonded alkenyl radicals in each molecule. Suitable alkenyl radicals contain from 1 to 10 carbon atoms and are exemplified by vinyl, allyl and 5-hexenyl. The silicon-bonded organic groups other than alkenyl radicals present in ingredient A are typically monovalent hydrocarbon and halogenated hydrocarbon radicals exemplified by alkyl radicals such as methyl, ethyl and propyl; aryl radicals such as phenyl; and halogenated alkyl radicals such as 3,3,3-trifluoropropyl. The molecular structure of ingredient A is typically linear, however there can be some branching due to the presence of trivalent siloxane units within the molecule. To achieve a useful level of tensile properties in the elastomers prepared by curing the present compositions, the number average molecular weight of this ingredient should be sufficient to achieve a viscosity at 25°C. greater than 0.1 Pa·s. The upper limit for the number average molecular weight of ingredient A is not specifically restricted and is typically limited only by the processability of the curable organosiloxane composition. The polyorganosiloxanes range from pourable liquids to gum type polymers that are typically characterized by Williams Plasticity values. Preferred embodiments of ingredient A are polydiorganosiloxanes represented by the general formula I where each R 8 is individually selected from monovalent hydrocarbon radicals and monovalent halohydrocarbon radicals, R 9 represents a vinyl or other alkenyl radical and n represents a degree of polymerization equivalent to a viscosity of at least 100 centipoise (0.1 Pa·s), preferably from 0.1 to 10 Pa·s. The two R 8 substituents on each of the silicon atoms in formula I can be identical or different and can contain from 1 to 20 carbon atoms. A range of from 1 to 10 carbon atoms is preferred based on the availability of the corresponding monomers. Most preferably, at least one of the hydrocarbon radicals on each silicon atom is methyl and any remainder are vinyl, phenyl and/or 3,3,3-trifluoropropyl. This preference is based on the availability of the reactants typically used to prepare the polydiorganosiloxane and the properties of cured elastomers prepared from these polydiorganosiloxanes. For the same reasons, R 9 is preferably vinyl or 5-hexenyl. Representative embodiments of ingredient A containing ethylenically unsaturated hydrocarbon radicals only at the terminal positions include dimethylvinylsiloxy-terminated polydimethylsiloxanes, dimethylvinylsiloxy-terminated polymethyl-3,3,3-trifluoropropylsiloxanes, dimethylvinylsiloxy-terminated-dimethylsiloxane/3,3,3-trifluoropropylmethylsiloxane copolymers and dimethylvinylsiloxy-terminated-dimethylsiloxane/methylphenylsiloxane copolymers. Methods for preparing ingredient A of the present compositions are by hydrolysis and condensation of the corresponding halosilanes or by condensation of cyclic polydiorganosiloxanes. These are sufficiently disclosed in the patent and other literature that a detailed description in this specification is not necessary. For applications requiring high levels of physical properties such as tear strength, it may be desirable to include in the curable organosiloxane composition a second polydiorganosiloxane containing ethylenically unsaturated hydrocarbon radicals bonded to both terminal and non-terminal silicon atoms. The curable organosiloxane compositions of this invention contain at least one organohydrogensiloxane that functions as a crosslinker for ingredient A. In the presence of the hydrosilation catalyst, ingredient C, the silicon-bonded hydrogen atoms of ingredient B undergo an addition reaction, referred to as hydrosilation, with the silicon-bonded alkenyl groups in ingredient A, resulting in crosslinking and curing of the composition. Ingredient B must contain at least 2 silicon-bonded hydrogen atoms in each molecule. If ingredient A contains only two alkenyl radicals per molecule, ingredient B must also contain an average of more than two silicon-bonded hydrogen atoms to achieve a crosslinked structure in the final cured product. The silicon-bonded organic groups present in ingredient B are selected from the same group of monovalent hydrocarbon and halogenated hydrocarbon radicals as the organic groups of ingredient A, with the proviso that the organic groups in ingredient B must be substantially free of ethylenic or acetylenic unsaturation. The molecular structure of ingredient B can be straight chain, branch-containing straight chain, cyclic or network. While the number average molecular weight of ingredient B is not specifically restricted, viscosities in the range of 3 to 10,000 centipoise (0.003 to 10 Pa·s) at 25°C. are preferred. The concentration of ingredient B is sufficient to provide a molar ratio of silicon-bonded hydrogen atoms to alkenyl radicals in the curable composition of from 0.5 to 20. A range of from 0.5 to 2 is preferred. When the curable composition contains less than 0.5 moles of silicon-bonded hydrogen atoms per mole of alkenyl radicals the composition cannot be satisfactorily cured. Bubble formation resulting from the generation of hydrogen gas can occur when the composition contains more than 20 silicon-bonded hydrogen atoms per alkenyl radical. Curing of the present compositions is catalyzed by a hydrosilation catalyst that is a metal from the platinum group of the periodic table or a compound of such a metal. These metals include platinum, palladium and rhodium. Platinum and platinum compounds are preferred based on the high activity level of these catalysts in hydrosilation reactions. Examples of preferred curing catalysts include platinum black, platinum metal on various solid supports, chloroplatinic acid, alcohol solutions of chloroplatinic acid and complexes of chloroplatinic acid with liquid ethylenically unsaturated compounds such as olefins and organosiloxanes containing ethylenically unsaturated hydrocarbon radicals bonded to silicon. Complexes of chloroplatinic acid with the aforementioned organosiloxanes containing ethylenically unsaturated hydrocarbon radicals are described in US-A 3,419,593, which teach our preferred catalysts. The concentration of ingredient C is equivalent to a platinum concentration of from 0.1 to 500 parts by weight of platinum metal, preferably from 1 to 50 parts by weight of platinum metal, per million parts (ppm), based on the combined weight of ingredients A and B. Curing does not proceed satisfactorily at below 0.1 ppm of platinum, while using more than 500 ppm results in no appreciable increase in cure rate and is therefore uneconomical. Mixtures of the aforementioned ingredients may begin to cure at ambient temperature. To obtain a longer working time or "pot life", the activity of the catalyst under ambient conditions can be retarded or suppressed by addition of a suitable inhibitor. Known platinum catalyst inhibitors include the acetylenic compounds disclosed in US-A 3,445,420. Acetylenic alcohols such as 2-methyl-3-butyn-2-ol constitute a preferred class of inhibitors that will suppress the activity of a platinum-containing catalyst at 25°C. Compositions containing these catalysts typically require heating at temperatures of 70°C. or above to cure at a practical rate. If it desired to increase the pot life of a curable composition under ambient conditions, this can be accomplished using an alkenyl substituted siloxane of the type described in US-A 3,989,667. Cyclic methylvinylsiloxanes are preferred. Inhibitor concentrations as low as one mole of inhibitor per mole of platinum will in some instances impart satisfactory storage stability and cure rate. In other instances, inhibitor concentrations of up to 500 moles of inhibitor per mole of platinum are required. The optimum concentration for a given inhibitor in a given composition can readily be determined by routine experimentation and does not constitute part of this invention. Some compositions may begin to cure under ambient conditions even when an inhibitor is present. One way to ensure storage stability is to package the ingredients of the curable composition in two or more containers, with the hydrosilation catalyst and the organohydrogensiloxane in separate containers. The contents of the containers are combined when it is desired to cure the composition. One-part organosiloxane compositions having excellent long-term storage stability at temperatures of 50°C. or greater can be prepared by first microencapsulating the platinum-containing hydrosilation catalyst in a thermoplastic or thermosetting polymer. Curable organosiloxane compositions containing microencapsulated hydrosilation catalysts are described in US-A 4,766,176 and US-A 5,017,654. The teaching of these patents relate to storage stable one-part organosiloxane compositions in which form the claimed compositions may be adapted. Suitable encapsulants include organosilicon resins and organic resins derived from ethylenically unsaturated hydrocarbons and/or esters of ethylenically unsaturated carboxylic acids such as acrylic and methacrylic acids. To achieve high levels of tear strength and other physical properties that characterize some types of cured elastomers which can be prepared using the compositions of this invention, it may be desirable to include a reinforcing filler such as finely divided silica. Silica and other reinforcing fillers are often treated with one of more of the known filler treating agents to prevent a phenomenon referred to as "creping" or "crepe hardening" during processing of the curable composition. Finely divided forms of silica are preferred reinforcing fillers. Colloidal silicas are particularly preferred because of their relatively high surface area, which is typically at least 50 square meters per gram. Fillers having surface areas of at least 300 square meters per gram are preferred for use in the present method. Colloidal silicas can be of the precipitated or a fume type. Both types of silica are commercially available. The amount of finely divided silica or other reinforcing filler used in the present compositions is at least in part determined by the physical properties desired in the cured elastomer. Liquid or pumpable polyorganosiloxane compositions typically contain from 10 to 60 percent by weight of silica, based on the weight of polydiorganosiloxane. This value is preferably from 30 to 50 percent. The filler treating agent can be any of the low molecular weight organosilicon compounds disclosed in the art as being suitable for preventing creping of organosiloxane compositions during processing. The treating agents are typically liquid hydroxyl terminated polydiorganosiloxanes containing an average of from 2 to 20 repeating units per molecule and organosilicon compounds such as hexaorganodisiloxanes and hexaorganodisilazanes that hydrolyze under the conditions used to treat the filler to form compounds with silicon-bonded hydroxyl groups. Preferably at least a portion of the silicon bonded hydrocarbon radicals present on the treating agent are identical to a majority of the hydrocarbon radicals present in ingredients A and B. A small amount of water can be added together with the silica treating agent(s) as a processing aid. It is believed that the treating agents function by reacting with silicon-bonded hydroxyl groups present on the surface of the silica or other filler particles to reduce interaction between these particles. When a silica filler is used, it is preferably treated in the presence of at least a portion of the other ingredients of the present compositions by blending these ingredients together until the filler is completely treated and uniformly dispersed to form a homogeneous material. The ingredients that are present during treatment of the filler typically include the silica treating agents and at least a portion of the polydiorganosiloxane(s) referred to herein as ingredient A. The present organosiloxane compositions can contain one or more additives that are conventionally present in curable compositions of this type to impart or enhance certain physical properties of the cured composition in addition to adhesion or to facilitate processing of the curable composition. Typical additives include non-reinforcing fillers such as quartz, alumina, mica and calcium carbonate; pigments such as carbon black and titanium dioxide; dyes, flame retardants and heat and/or ultraviolet light stabilizers. Resinous organosiloxane copolymers can be used in place of or in combination with one or more reinforcing fillers to improve the physical properties of the cured organosiloxane composition. A preferred type of resinous copolymer contains repeating units of the general formula SiO 4/2 in addition to triorganosiloxy units of the general formulae R 10 3 SiO 1/2 and diorganovinylsiloxy units of the general formula CH 2 =CH(R 11 ) 2 SiO 1/2 . In these formulae, R 10 and R 11 are individually monovalent hydrocarbon or substituted monovalent hydrocarbon radicals as previously defined for the R 8 radicals of ingredient A. The molar ratio of the combination of triorgano-siloxy units and diorganovinylsiloxy units to SiO 4/2 units in the resinous copolymer is from 0.7 to 1.2, inclusive. The vinyl-containing units constitute from 2 to 8 percent by weight of the copolymer, which preferably contains at least two vinyl radicals per molecule. In preferred embodiments of the copolymer, the ranges for the molar ratio of diorganovinylsiloxy: triorganosiloxy: SiO 4/2 units is 0.08-0.1 : 0.06-1 : 1. The resinous copolymers can be prepared as described in US-A 2,676,182. The copolymers described therein contain from 2 to 23 percent by weight of hydroxyl groups, which is considerably above the maximum level of 0.8 weight percent preferred for precursors of the present copolymers. The hydroxyl content of the precursor can be conveniently reduced to the desired level by employing a higher concentration of triorganosiloxane units than the concentration range taught by that document. Briefly, the method of US-A 2,676,182, comprises reacting a silica hydrosol under acidic conditions with the appropriate amount of hexamethyldisiloxane or trimethylchlorosilane. The resinous copolymers used to prepare the present elastomers can be obtained by reacting a product of US-A 2,676,182 with the required amount of a hexaorganodisilazane or a hexaorganodisiloxane wherein each silicon atom contains a vinyl radical and two methyl or other hydrocarbon radicals represented by R 1 and R 2 in the foregoing formula. The compositions of this invention can be prepared by combining all of ingredients at ambient temperature. Any of the mixing techniques and devices described in the prior art can be used for this purpose. The particular device used will be determined by the viscosity of the ingredients and the final curable composition. Suitable mixers include but are not limited to paddle type mixers, kneader type mixers and two- and three-roll rubber mills. Cooling of the ingredients during mixing may be desirable to avoid premature curing of the composition. The concentration of the adhesion-promoting composition required to achieve adeqate adhesion between the cured organosiloxane composition and a substrate is dependent upon a number of variables. These include the reactants present in the curable composition, the substrate to which adhesion is desired and the conditions under which ahesion is to be maintained. For many end-use applications, adequate adhesion can be achieved when the adhesion-promoting composition constitutes from 0.5 to 2 percent of the total weight of the curable organosiloxane composition. When the curable compositions are used as adhesives or coatings for electronic devices the concentration of adhesion-promoting additive is 1 weight percent. To maximize storage stability the curable compositions are preferably kept in closed containers until used. If greater storage stability is desired, the compositions can be packaged in two or more containers with the organohydrogensiloxane (ingredient B) and the platinum group metal catalyst in separate containers. Depending upon the types and concentrations of ingredients A and B, cured organosiloxane materials prepared using the present compositions can vary in properties from brittle resins to elastomers to gels. They are useful in a variety of end-use applications as coatings or as molded or extruded articles. Unfilled materials are particularly useful as adhesives, protective coatings, encapsulants and potting compositions. They are especially effective for protecting delicate electronic devices such as transistors and integrated circuits from damage by moisture and other materials present in the environment that can adversely affect operation of the device. The compositions can be used to coat either the individual devices or a circuit board containing a number of these devices together with other electronic components. The present compositions can be applied to substrates by spraying, dipping, extrusion or by the use of a brush, roller or coating bar. The selection of a particular application method will be determined at least in part by the viscosity of the curable composition. The viscosity of the composition can be reduced using suitable solvents or reactive diluents as known in the art. Curing of the present compositions commences when ingredients A, B and C are combined. One advantage of our adhesion promoting compositions is the absence of substantial retardation of the curing reaction. A second advantage of these compositions is their ability to cohesively bond to a variety of organic and inorganic substrates at curing temperatures as low as 25°C. The compositions are therefore suitable for application to substrates that cannot withstand the elevated temperatures of 100°C. or higher required to cure organosiloxane compositions containing prior art adhesion promoters such as epoxide compounds that inhibit platinum group metal catalysts. Preferred compositions cure over a period of several hours under ambient conditions. As is true for other compositions that cure by a platinum-catalyzed hydrosilation reaction, curing can be accelerated by heating. Curing temperatures of from 25 to 80°C. are preferred. The following examples describes preferred one-part curable compositions of the present invention. Unless otherwise specified all parts and percentages in the example are by weight and viscosities were measured at 25°C. Example 1 (Reference) This example demonstrates the high level of adhesion imparted by the present additives. Curable compositions were prepared by blending the following ingredients to homogeneity: as ingredient A, 98 parts of a dimethylvinylsiloxy-terminated polydimethylsiloxane having a viscosity of 0.4 Pa·s; as ingredient B, 6 parts of a trimethylsiloxy-terminated polydiorganosiloxane having an average of five methylhydrogensiloxane units and three dimethylsiloxane units per molecule with a silicon-bonded hydrogen atom content in the range from 0.7 to 0.8 weight percent; as ingredient C, 0.2 part of a reaction product of hexachloroplatinic acid and sym-tetramethyldivinyldisiloxane that had been diluted with a liquid dimethylvinylsiloxy terminated polydimethylsiloxane in an amount sufficient to achieve a platinum content of 0.7 weight percent; 93 parts of a finely divided quartz; 0.9 part of zinc oxide; 0.5 part of carbon black; as the hydrosilation catalyst inhibitor, 0.5 part of cyclic methylvinylsiloxanes; 0.05 part of aluminum acetylacetonate; and 2 parts of adhesion promoting additive. Three adhesion promoting additives were prepared by heating mixtures of reactants for 2 hours at 80°C. The resultant mixtures were then heated under reduced pressure to remove volatile materials. The reactants were the diallyl ether of trimethylolpropane (D1), 3-glycidoxypropyltrimethoxysilane (D)(2)(i), tetraethyl orthosilicate (D)(2)(ii) and 1.5 parts of tetrabutyl titanate. The parts by weight of the reactants and the temperature or pressure used to remove volatile materials from the resultant adhesion promoting additive are recorded in Table 1. All additives were prepared using 1.5 parts of tetrabutyl titanate. The substrates on which the curable compositions were evaluated were strips of material measuring 1 x 4 inches (2.5 x 10 cm.). The materials were aluminum, copper, glass and FR4 type printed circuit board. The strips were cleaned by rubbing them for ten strokes with a cheesecloth that had been soaked in methyl isobutyl ketone and then allowing the substrates to dry under ambient conditions. Test samples were prepared using two strips of a substrate separated by a 1/16 inch (1.6 mm)-thick Teflon(R) spacer containing a 1/2 inch (1.3 cm)-diameter hole in its center. The spacer was placed in the center of one of the strips and the hole was filled with a curable composition containing one of the three adhesion promoting additives. A second strip of the same material was then placed on top of the spacer such that its longitudinal axis was perpendicular with respect to the longitudinal axis of the lower strip. The resulting composite was clamped together and the organosiloxane composition in the spacer was cured by heating the composite for 2 hours at 70°C. Each strip was then placed in the one of the jaws of a tensionometer and the composite was subjected to sufficient force (F) to pull the strips apart. The extent of cohesive failure (% CF) was determined by observing the percentage of the substrate area to which cured composition adhered relative to the area that had been in contact with the curable composition. The results are recorded in Table 2. The data in Table 2 indicate that the three adhesion promoting additives imparted equivalent degrees of adhesion at a cure temperature of 70°C. All samples exhibited 100 percent cohesive failure and similar adhesive strengths (F values). Example 2 This example demonstrates the superior adhesion at lower curing temperatures achieved using adhesion promoting additives prepared using both an epoxy-substituted organosilicon compound and a tetraalkyl orthosilicate. Curable organosiloxane compositions were prepared and evaluated for adhesion as described in Example 1, with the exception that the compositions were cured for 2 hours at 50°C. Some of the samples were tested the same day and others were allowed to remain under ambient conditions for up to eight days, during which they were evaluated. The time interval between curing and testing and the test results are recorded in Table 3. Additive No. 1 Days Following Curing Substrate Al Cu FR4 Glass % CF/F (MPa) 1 100/1.4 0/1.2 100/1.1 0/0.4 2 100/1.3 0/0.97 100/1.5 100/1.3 5 100/1.6 100/1.4 100/1.4 100/1.5 8 100/1.3 100/1.1 100/1.1 100/1.3 Additive No. 2 Days Following Curing Substrate Al Cu FR4 Glass % CF/F (MPa) 1 0/0.7 0/0.5 0/0.6 40/1.2 2 0/1.0 0/0.1 0/0.5 100/1.2 4 0/0.5 0/0.8 0/0.9 100/1.3 7 0/0.9 0/0.5 0/0.7 100/1.1 Additive No. 3 Days Following Curing Substrate Al Cu FR4 Glass % CF/F (MPa) 1 0/0.7 0/0.2 0/0.5 0/0.3 2 0/0.3 0/0.7 0/1.2 0/0.6 4 0/0.03 0/0.6 100/1.2 0/0.3 7 0/0.1 0/0.1 20/1.2 0/0.6 Additive 2 did not achieve adhesion to aluminum, copper or FR4 board at a cure temperature of 50°C. even after aging. Adhesion additive 3 did not adhere to any of the substrates. Only additive 1, which was prepared according to this invention, exhibited 100 percent cohesive failure to all substrates, both immediately after being cured at 50°C. and following aging. Example 4 This example demonstrates the requirement for prereacting the ingredients of the present adhesion promoting additives prior to blending them into a curable organosiloxane composition. Curable compositions and test samples were prepared as described in Example 1, with the exception that reactants were not reacted prior to being added to the other ingredients of the curable organosiloxane compositions. They were then cured for 2 hours at 50°C. The results of the adhesion evaluation are summarized in Table 5. Some of the cured samples were allowed to age under ambient conditions as described in the preceding examples prior to being evaluated. Unreacted Ingredients of Additive No. 1 Days Following Curing @50°C. Substrate Al Cu FR4 Glass % CF/F (MPa) 1 0/0.8 40/0.8 2 0/0.9 10/0.7 4 0/0.9 0/0.3 75/1.0 7 60/0.9 20/0.4 75/1.0 The data in Table 5 demonstrate that in addition to not exhibiting the degree of adhesion achieved by compositions containing the pre-reacted ingredients of the present adhesion promoters (Table 2), the presence of these unreacted ingredients inhibits curing of compositions on copper and FR4 printed circuit boards. Example 5 This example demonstrates the effect of prereaction on the ingredients of an adhesion promoter composition of this invention relative to an adhesion promoter composition containing 3-methacryloxypropyltri methoxysilane in place of an ethylenically unsaturated ether of a polyhydric alcohol. Curable compositions were prepared using the ingredients described in Example 1 with the exception that n-propyl orthosilicate was used instead of the alkyl orthosilicate ingredient. Ingredient D1 was also replaced with 3-methacryloxypropyltrimethoxysilane (MPTMS) in the comparative example. The parts by weight of the adhesion promoting additives and the temperature and pressure used to remove volatile materials from the resultant pre-reacted additive are recorded in Table 1. For comparative purposes the same amounts of adhesion promoting additives were added directly to the other ingredients of the curable composition without being pre-reacted. Additive No. (D)(1) (D)(2)(i) (D)(2)(ii) MPTMS Temp. (°C.) Pressure (mm Hg) 1 0.29 0.98 0.87 0 90 90 2 0 0.25 1.1 0.15 90 87 Additive 1 contained 0.04 part of tetrabutyl titanate and additive 2 contained 0.1 part of that catalyst. Curable organosiloxane compositions were prepared and evaluated for adhesion as described in Example 1, with the exception that the compositions were cured for 4 days and 7 days at 25°C. The force required to separate the substrates was measured in kilograms and not all of the compositions contained aluminum acetylacetonate (AlAcAc). This catalyst was used at a concentration of 0.05 percent, based on the weight of the curable composition. The cure times and the test results are recorded in Table 7. %CF represents the percent cohesive failure, i.e. the percentage of the substrate surface to which the curable composition adhered following separation of the substrates and the force required to achieve this separation is reported in kilo-pascals. The substrates used for this evaluation were aluminum, copper, FR4 board and glass. Additive No. 1 (Pre-Reacted) Days @ 25°C. % CF/F (KPa) Al Cu FR4 Glass 4 (no AlAcAc) 0/139.5 0/179.4 100/260.3 0/247.1 7 (no AlAcAc 0/136.1 0/68.4 100/255.2 0/110.5 7 (w. AlAcAc) 100/169.9 100/197.0 100/230.1 100/344.1 Additive No. 1 (Not Pre-Reacted) 4 (no AlAcAc) 0/33.5 0/72.6 0/231 0/98.4 7 (no AlAcAc 0/25.8 0/89.3 100/279 0/129.1 7 (w. AlAcAc) 90/186.7 0/170.6 75/174.8 100/274.1 Additive No. 2* (Pre-Reacted) 4 (no AlAcAc) 0/114.5 0/62.1 100/13.7 0/76.7 7 (no AlAcAc 0/98.4 0/56.5 0/167.8 0/105.6 7 (w. AlAcAc) 50/25.3 0/107.7 100/204.9 0/125.8 Additive No. 2* (Not Pre-Reacted) 4 (no AlAcAc) 0/143.8 0/161.2 0/235.2 0/129.1 7 (no AlAcAc 0/96.3 0/161.2 50/234.3 0/152.4 7 (w. AlAcAc) 10/200 0/207 100/180.4 100/230.8 The data in Table 7 demonstrate that after curing for 4 days at 25°C. without aluminum acetylacetonate, compositions containing the pre-reacted adhesion promoting ingredients of this invention (additive no. 1) had better adhesion than the composition containing the same adhesion promoting ingredients that had not been pre-reacted. Under the same conditions compositions containing the unreacted ingredients of the comparative adhesion additive (additive no. 2) had better adhesion than the composition containing the same adhesion promoting ingredients that had been pre-reacted. The data also demonstrate the beneficial effect of aluminum acetylacetonate in improving the adhesion imparted by the present pre-reacted adhesion promoting additives (100% cohesive failure on all substrates). The corresponding composition containing the unreacted adhesion promoting ingredients achieved 100% cohesive failure only to glass. Only on FR4 board did the comparative adhesion promoter containing reacted ingredients achieve 100 percent cohesive failure, with better bond strength than the corresponding composition in which the adhesion promoting ingredients had not been pre-reacted. The composition containing the unreacted adhesion promoting ingredients had 100% cohesive failure on all four substrates with higher bond strength to aluminum, copper and glass.

Claims: An adhesion-promoting additive for curable organosiloxane compositions, said additive comprising the reaction product of (1) a partial ether derived from an ethylenically unsaturated alcohol and a polyhydric alcohol, (2) at least one organosilicon compound containing at least one epoxide group and at least one silicon-bonded hydrolyzable group, and (3) at least one tetraalkyl orthosilicate; where said reaction product is prepared in the presence of a catalytically effective amount of an organotitanium compound. An additive according to claim 1 where the terminal carbon atoms of said alcohol are unsaturated; said polyhydric alcohol contains from 2 to 10 carbon atoms; said organosilicon compounds correspond to the formula R 1 SiR 2 m R 3 (3-m) , where R 1 represents an epoxyalkyl radical or an epoxy-substituted cycloalkyl radical, R 2 represents an alkyl radical, R 3 represents a hydrolyzable group and m is 0 or 1, and said tetraalkyl orthosilicate is selected from tetramethyl orthosilicate, tetraethyl orthosilicate and tetra-n-propyl orthosilicate. An additive according to claim 2 where R 3 is alkoxy, carboxy or ketoximo, said unsaturated alcohol is allyl alcohol, said polyhydric alcohol is selected from ethylene glycol, propylene glycol, glycerol, trimethylolpropane and pentaerythritol, said organosilicon compound and said tetraalkyl silicate each constitute at least 20 weight percent of the reactants used to prepare said additive, the organotitanium compound constitutes from 0.1 to 5 percent of the combined weight of the reactants used to prepare said additive and said additive further comprises a chelated aluminum compound. An additive according to claim 3 where said ether is a mono- or di-allyl ether of trimethylolpropane, said organosilicon compounds are monoepoxytrialkoxysilanes wherein R 1 is 3-glycidoxypropyl or epoxycyclohexylmethyl and said tetraalkyl orthosilicate is tetramethyl orthosilicate or tetraethyl orthosilicate. A curable organosiloxane composition exhibiting adhesion to both organic and inorganic substrates following curing at temperatures below 100°C., said composition comprising (A) a polyorganosiloxane containing at least two alkenyl radicals per molecule, (B) an amount sufficient to cure said composition of an organohydrogensiloxane containing at least two silicon-bonded hydrogen atoms per molecule, where the sum of the alkenyl radicals per molecule of (A) and the silicon-bonded hydrogen atoms per molecule of (B) is greater than 4 and wherein the molar ratio of silicon-bonded hydrogen atoms to alkenyl radicals in the curable composition is from 0.5 to 20, (C) an amount sufficient to promote curing of said composition of a hydrosilation catalyst selected from metals of the platinum group of the periodic table and compounds of said metals, and (D) an amount sufficient to achieve adhesion to substrates in contact with said composition during curing at temperatures below 100°C. of the adhesion promoting additive of claim 1. A composition according to claim 5 where the terminal carbon atoms of said unsaturated alcohol are unsaturated; said polyhydric alcohol contains from 2 to 10 carbon atoms; the silicon compounds used to prepare said reaction product include at least one of said organosilicon compounds and at least one of said tetraalkyl orthosilicates, said organosilicon compounds correspond to the formula R 1 SiR 2 m R 3 (3-m) , where R 1 represents an epoxyalkyl radical or an epoxy-substituted cycloalkyl radical, R 2 represents an alkyl radical, R 3 represents a hydrolyzable group and m is 0 or 1, said tetraalkyl orthosilicate is selected from tetramethyl orthosilicate, tetraethyl orthosilicate and tetra-n-propyl orthosilicate; said curable polyorganosiloxane is a polydiorganosiloxane corresponding to the formula where each R 8 is individually selected from monovalent hydrocarbon radicals and monovalent halohydrocarbon radicals, R 9 represents an alkenyl radical and n represents a degree of polymerization equivalent to a viscosity of at least 0.1 Pa·s; and said hydrosilation catalyst is a platinum compound and is present at a concentration equivalent to from 0.1 to 500 parts by weight of platinum per million parts, based on the combined weight of said polyorganosiloxane and said organohydrogensiloxane. A method for bonding two substrates, the method comprising the sequential steps of applying to a surface of one of said substrates the curable organosiloxane composition of claim 5 and placing the surface coated with said composition in contact with a surface of a second substrate and then curing said organosiloxane composition.

Inventors: BILGRIEN CARL JOSEPH, BILGRIEN, CARL JOSEPH
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1994
Main classification: C08G77
IPC codes: C08G77, C09J183, C08K3, C09J5, C08L83, H05K3